1. Field of the Invention
The present invention relates to a preparation method of polymeric rod and gradient index rod lens using free radical bulk polymerization with temperature gradient. More particularly, the present invention relates to a method for preparing polymeric rod having nearly perfect outer shape by free radical bulk polymerization with temperature gradient which enables significant decrease of transmission loss of polymeric rod caused by scattering, and a method for preparing a polymeric gradient-index rod lens employed as an optical component.
2. Description of the Prior Art
A polymeric gradient-index rod lens is generally employed in connection of optical fibers or connection between laser diode and optical fiber since it has an advantage of lowering connection loss.
Such polymeric gradient-index rod lens may be prepared from glass or polymer. Particularly, when using polymer, it is possible to produce polymeric gradient-index rod lens having various gradient-index due to diverse polymer types, while avoiding toxic preparation condition which has been a problem when using glass. Moreover, it is another advantageous aspect of using polymers to operate the process in a relatively low temperature of about 100.degree. C., compared by excessively high temperature of 300.degree. to 700.degree. C. which is inevitable when using glass. For these reasons, recent studies have been focused on the preparation method using polymers see Y. Koike, in Polymers for Lightwave and Integrated Optics, Edited by L. A. Hornak, Dekker (1993)!.
Since a polymeric gradient-index of gradient-index rod lens largely depends on the geometric shape while the coarse surface and internal defects thereof may cause an transmission loss due to a scattering, it is essential to obtain a polymeric rod having perfect outer shape without internal flaws in order to prepare the above-mentioned polymeric gradient-index rod lens.
Almost all of the conventional preparation methods of a gradient-index rod lens use a free radical bulk polymerization in forming polymeric rod see Y. Koike et al., Appl. Opt., 24(24):4321-4325, 1985!.
Although free radical bulk polymerization is advantageous in that the resultant polymer is not nearly contaminated as it is prepared directly from monomers, it is hard to control its polymerization rate due to an exothermic reaction, high activation energy and the difficulty in heat dissipation by gel effect, etc.
Also, as shown in Table 1, the density of polymer is higher than that of monomer, since the volume of a polymer decreases as a conversion of monomer into polymer proceeds and bubbles are generated at that amount. Such bubbles not only affect geometric shape of the polymeric rod but also cause transmission loss when they remain inside of the rod.
Despite said problems, there have been little effort in the prior art method for preparation of polymeric gradient-index rod lens to effectively remove the bubbles which are generated by volume change.
TABLE 1 ______________________________________ Densities of the representative monomers and polymers.sup.# Density (g/cm.sup.3) Volume (25.degree. C.) change Monomers monomers polymers (%) ______________________________________ vinyl chloride 0.919 1.406 34.4 acrylonitrile 0.800 1.17 31.0 vinylidene chloride 1.213 1.71 28.7 methyl acrylate 0.952 1.223 22.1 methyl methacrylate 0.940 1.179 20.6 styrene 0.905 1.062 14.6 ______________________________________ .sup.# G. Odian, Principles of Polymerization, 3rd Ed., John Wiley & Sons Inc., 1991.
Generally, as conversion of monomers into polymer proceeds, the viscosity increases gradually or rapidly. As seen in the attached FIG. 1, when monomers are homogeneously heated to proceed radical polymerization in the entire monomer solution, bubbles (3) generated by the density difference between monomer and polymer are fixed by prepolymer (1) or gel (2), and they cannot be released into the atmosphere and inevitably remained inside of the polymers. As higher the polymerization temperature, greater the addition amount of cross linking agent, and smaller the inner diameter, the amount of the residual bubbles present inside the polymers becomes large.
Meanwhile, as one of the most recent methods of preparing gradient-index rod lens, it is disclosed that a mixture of at least two monomers and polymerization initiator having different reaction rate and refractive index is placed in an optically transparent reaction tube and the photocopolymerization is carried out by moving the UV source upward in a vertical line while rotating the reaction tube see: Y. Ohtsuka and I. Nakamoto, Appl. Phys. Lett., 29(9): 559-561, 1976!.
However, the process of the aforementioned prior art is cumbersome since the reaction tube must be rotated. Another problem is that there are many possibilities of bubble generation during the procedure of completing polymerization of prepolymer or gel by heating to obtain solid polymer, which is common in the conventional method employing photocopolymerization reaction.